The present invention relates to non-invasive detectors for detecting the presence of pieces of equipment through the walls of pipes, particularly steel pipes, typically pipes that are used in oil wells and the like. More specifically, it relates to the circuitry of such detectors.
In the oil industry, it is common to retain moving equipment inside a pressure vessel. An example is a downhole instrument which is retrieved from a well through a riser. The problem is that the pressure vessel forming the top of the riser is usually made of steel, and this stops the operator of the equipment from seeing what is going on because he cannot see through steel. Accordingly, some sort of sensor is required, so that the equipment operator can detect what is going on inside the pressure vessel without having to open it. This has safety benefits because it may prevent an accident and it has operational benefits because it may allow the operator to position equipment more accurately.
Various sensing devices have been proposed. The traditional sensing device is a sensing coil which encircles the riser, and detects magnets attached to equipment which may be passing up or down the well. As an alternative to sensing coils, other sensing devices can be used. Thus U.S. Pat. No. 5,323,856 (Halliburton) suggests a variety of detectors, such as the use of Hall effect, fiber optic, or Faraday effect detectors.
U.S. Pat. No. 3,843,923 (Stewart and Stevenson) is a more detailed example of the use of Hall effect devices. To detect the movement of a pipe joint through a pipe, a locator comprises a ring magnet with a pair of detector rings mounted one on each side (ie one above the ring magnet and one below it). Each detector ring comprises a set of four Hall effect devices mounted around the pipe. The Hall effect devices of a set have their outputs summed, and the sums of the two sets are differenced.
Further, our own earlier GB patent application no. 01.22431.0 (filed Sep. 17, 2001) describes the use of two pairs of Hall effect devices, one pair being located opposite each other on the riser and the other pair being similarly located opposite each other and longitudinally below the first pair. This patent application notes that with sufficient sensitivity, devices can be detected passing through the well even though they do not have magnets attached to or incorporated in them, provided that they use magnetizable material and their passage produces a sufficient change in the ambient magnetic field (ie the field due to drilling rig steelwork, the earth""s field, etc).
The general object of the present invention is to provide improved detector circuitry suitable for use with oil well pipes and risers.
According to the invention, there is provided detector circuitry for detecting magnetic field disturbances resulting from the movement of equipment through a pipe of magnetic material, the detector circuitry comprising a difference circuit fed with the outputs of two longitudinally spaced sensor means via respective DC blocking means, a saturating difference amplifier feeding an integrator and fed with the outputs of the difference circuit and the integrator, and indicating means fed from the integrator.
It may also be desirable for the sensor electronics to incorporate a threshold level setting. This can be used as a comparator signal to give a warning when a change in signal level is detected. Also, by having a threshold level that can be adjusted, it is possible to avoid detections that may result from sensor or system noise. A second comparator may be connected in parallel with the first and fed with inverse or reversed inputs, so that signals of both polarities can be detected. The indicating means may include matching means.